Dielectrophoresis (DEP) is the analog of optical tweezers that are capable of manipulating objects, cells, and even a single molecule in an aqueous solution (P. J. Burke, Nano-dielectrophoresis: Electronic nanotweezers, 2003, Encyclopedia of Nanoscience and Nanotechnology, American Scientific). DEP describes induced particle motion along an electric field gradient due to the interaction of the induced dipole in the particles and the applied electric field (H. A. Phol, Dielectrophoresis, Cambridge University Press 1987). An analytical expression of DEP force is illustrated in FIG. 1 (T. B. Jones, Electromechanics of Particles, Cambridge University Press, 1995), where U is the volume of the object, the factor in parentheses is the RMS value of the electric field, and αr is the real part of the Clausius-Mosotti factor which relates the dielectric constant of the object ∈p and dielectric constant of the medium ∈m. The star (*) denotes that the dielectric constant is a complex quantity.
The term αr can have any value between 1 and −½, depending on the applied AC frequency and the dielectric constants of the object and medium. If αr less than zero, a particle will tend to move towards a lower electric field region. This is commonly referred to as negative DEP. On the other hand, if αr is positive, a particle will tend to move towards a higher electric field region. This is commonly referred to as positive DEP. DEP force is AC frequency dependent, so by varying the frequency of the applied AC bias, the force can be adjusted from positive to negative DEP, and vice versa. Thus, there are two modes in which DEP forces can operate: positive, in which substances are attracted to high electric field strength regions, and negative, in which substances are repelled by high electric field strength regions.
DEP has been used to manipulate objects (N. G. Green, et al., J. Phys. D., 1997, 30, 2626-2633), to separate viable/non-viable yeast (G. H. Markx, et al., J. Biotechnology, 1994, 32, 29-37) and other micro-organisms such as separating Gram-positive bacteria from Gram-negative bacteria (G. H. Marks, et al., Microbiology, 1994, 140, 585-591), and to remove human leukemia cells and other cancer cells from blood (F. F. Becker, et al., J. Phys. D.: Appl. Phys., 1994, 27, 2659-2662; F. F. Becker, et al., Proc. Nat. Acad. Sci. (USA), 1995, 92, 860-864). The cells are manipulated by a traveling wave generated by a series of patterned electrodes lining up and charged with phase-shifted AC signals (A. D. Goater, et al., J. Phys. D., 1997, 30, L65-L69). The patterned electrodes can be patterned in an independently controlled array to provide such a traveling wave.
Optically activated DEP systems have been compiled using low-power laser light focused to induce DEP between two pattern-less surfaces, such as a indium tin oxide (ITO) transparent glass electrode and a substrate coated with photoconductive material to complete the circuit (P. Y. Chiou, et al., Cell Addressing and Trapping using Novel Optoelectric Tweezers, 2004, IEEE International Conference on Micro Electro Mechanical Systems, Technical Digest, 17th Maastricht, Netherlands, Jan. 25-29, 2004). A non-uniform field is created by a well-defined laser spot, and the objects in the liquid layer in between the two electrodes move towards or away from the illuminated spot by the negative or positive dielectrophoretic force. Silicon nitride coats the photoconductive material to provide separation between the photoconductive material and the liquid layer. Typical light activated DEP relies on a transparent ITO electrode to permit a focused laser beam to pass through the ITO electrode and illuminate a photoconductor.
DEP, whether optically activated or electrically activated can be used to separate or manipulate uncharged objects and objects that have a charge such as DNA, and cells that have a net charge on their surface. Typically, metal electrodes are used in a uniform or non-uniform electric field to provide the driving force to separate or manipulate objects. Electrically activated EP relies on metal electrodes to generate uniform or non-uniform electric fields, providing the driving force to separate or manipulate charged objects. Optically activated EP can rely on a transparent metal or metallic electrode that can permit a light beam, for example, a focused laser, to pass through the electrode and illuminate a photoconductive material adjacent to a non-transparent electrode, generating a non-uniform electric field and providing the driving force to separate or manipulate charged objects.
It can be advantageous to be able to separate objects, including unstable objects like RNA, in a separation medium using a low energy light source. Such separations can be effective and efficient and, in the case of labeled substances, can provide a means for visualization.